With the ever growing use of electronic communication systems, it is necessary to ensure that only authorised users obtain access to restricted access data. For example, a computer may be accessible via the public telephone network by having a modem that links up, via the telephone network, with the modem of a user. Normally, the decision as to whether to permit access to any restricted access data in the computer is determined by whether the user transmits, via the modems, an approved access code or identity code to the computer. However, so-called "hackers" are able by detailed knowledge of existing systems or by persistent guessing of possible identity codes to gain access to restricted access data in computers.
Accordingly, there is the need for an arrangement in which the "hacker" is unaware that he is being vetted as to whether or not he is entitled to access to the restricted access data. Alternatively, if the "hacker" is aware that he is being vetted, he must be unable to work out how to circumvent the security features.